1. Field of the Invention
The present invention is generally related to a condensing tube and a filtration module thereof, and more particularly to condensing tube and a filtration module thereof for membrane distillation.
2. Description of the Prior Art
Membrane distillation is a distillation process to separate a high-temperature liquid and a low-temperature liquid by a porous membrane which has gas permeability but being liquid impermeable. Generally, there are four basic configurations for membrane distillation (MD): direct-contact membrane distillation (DCMD), air-gap membrane distillation (AGMD), sweeping-gas membrane distillation (SGMD), and vacuum membrane distillation (VMD). Among them, AGMD exhibits the highest thermal efficiency. AGMD has not only been applied for separation of non-volatile components from water like desalination as other MD configurations, but also been specifically suitable to separate some volatile substances which can not be removed in direct contact membrane distillation (DCMD), e.g. alcohols from an aqueous solution. However, the air gap between the membrane and the condensation surface results in lower permeate flux of AGMD since its configuration presents a new resistance to heat and mass transfer. According WO patent 86/07585A, an AGMD distillation apparatus is disclosed where it indicates that the thickness of the air gap should be between 0.2 mm and 1.0 mm to increase flow rate and reduce heat loss.
In searching the reports on the theory and experiments of AGMD, although some design and manufacturing reports about various membranes having high permeation can be found, the availability of the industrial MD modules is up to now one of the limitations for MD process implementation, even for AGMD. Due to the fact that MD is a non-isothermal process, the design of the MD modules must not only provide good flow conditions, low pressure drop and high packing density but also guarantee a good heat recovery function and thermal stability. Among various module configurations, a flat-sheet membrane in plate and frame modules (Liu, G. L.; Zhu, C.; Cheng, C. S.; Leung, C. W., Theoretical and experimental studies on air gap membrane distillation. Heat Mass Transf. 1998. 34, 329) shows the highest permeate flux, but the specific surface area of the membrane per module is the lowest. On the other hand, a hollow fiber module (Cheng, L. H.; Wu, P.-C.; Chen, J. Numerical simulation and optimal design of AGMD-based hollow fiber modules for desalination. Ind. Eng. Chem. Res. 2009, 48, 4948) has the highest packing density, but the pressure drop along the fiber is the biggest because of the high ratio of fiber length to the fiber diameter.
Therefore, a membrane distillation module or system having high enough permeate flux to reach the application needs is very important for industries.